In general, an automatic welding machine brings a large number of operating parameters into play relating to the welding method used, to the geometrical characteristics of the groove delimited between the two parts to be welded together, and to welding conditions (nature, shape and position of the parts, . . . ). In most industrial applications, a welding operation is performed in several passes, and obtaining a high quality weld depends on the accuracy with which the first pass is performed. This implies that the end surfaces to be welded together need to be machined to delimit a groove between them which is as regular as possible, and that a guidance system should be provided which is suitable for tracking the shape of the groove so that the welding is applied very accurately to the bottom of the groove, particularly during the first pass.
Thus, it is common practice to machine a chamfer on the two ends to be welded together. Depending on the angle given to the chamfer, the resulting groove is flared to a greater or lesser extent, and the wider the groove the greater the number of passes and the longer the welding time. Although a narrow groove avoids the above drawbacks, is not without difficulties for the guidance system, as described below.
In practice, even after such machining, the groove delimited by the two chamfers cannot have constant geometrical characteristics over its entire length. Once the two end surfaces have been brought face to face, the manufacturing tolerances that apply to the parts to be welded together and to the machining of the chamfers necessarily give rise to variations in the width and height of the groove. When the two parts to be welded together are tubes of considerable length, for example, with the chamfers being machined to a tolerance of .+-.0.1 mm, the resulting tolerance on the width of the groove is .+-.0.2 mm in conjunction with differences in level of 2 mm to 3 mm, due in particular, to ovalization of the tubes.
When the welding method is electric arc welding, and visible under a protective gas as is commonly used for welding tubes end-to-end, the welding electrodes carried by the electrode holders must penetrate into the groove and lie at a determined distance from the walls and the bottom of the groove or from the previous layer of welding. Under these conditions, the accuracy of the guidance system becomes of great importance, particularly when the welding method is used in a narrow groove.
Consequently, welding accuracy, particularly during the first pass, requires the welding electrodes to be accurately positioned not only relative to the midplane of the groove, but also relative to the bottom of the groove. The more accurate the guidance system, the closer these two positioning constraints are satisfied, with midplane positioning being sensitive to variations in groove width and with bottom positioning being sensitive to variations in difference in level.
In prior mechanical guidance systems, the running means are wheels which run on both sides or along one side of the groove, and the feeler members are also constituted by at least one wheel running along the inside of the groove and bearing simultaneously against both walls thereof. Experience shows that such feeler members are ill-adapted to a narrow groove and that they do not take account of variations in the width and in the difference in level of the groove.
An object of the invention is to provide a guidance system capable of positioning welding electrodes accurately inside the groove and capable of taking account of variations in width and differences in level thereof, thereby improving welding quality as is required in certain applications, in particular for welding lengths of tube end-to-end to construct pipelines for placing on the seabed.